Filter unit for filtering particles contained in exhaust gas of an internal combusting engine

ABSTRACT

The inventive filter unit for filtering particles contained in exhaust gas of an internal combusting engine comprises sets of imbricated input channels ( 10, 11 ) and adjacent output channels ( 12, 13 ) which are fluidly communicating by means of the lateral walls thereof. Said lateral walls are cross-sectionally provided with a corrugation which is determined in such a way that the total volume of the input channels ( 10, 11 ) is increased with respect to that of the output channels ( 12, 13 ), whereby the total volume (Ve) of the input channels ( 10, 11 ) being greater than that (Vs) of the output channels ( 12, 13 ).

The invention relates to a filter unit for filtering particles containedin the exhaust gas of an internal combustion engine, in particular ofthe diesel type, and to a filter body including at least one filter unitaccording to the invention.

Porous honeycomb structures are used as filter bodies for filteringparticles emitted by diesel vehicles. These filter bodies are generallymade of ceramic (cordierite, silicon carbide, etc.). They may bemonolithic or consist of separate units. In the latter case, the unitsare cemented together by means of a ceramic cement. The whole is thenmachined to the required section, which is generally circular orelliptical. The filter body includes a plurality of passages. It isinserted into a metal enclosure. Each passage is blocked at one or theother of its ends. There are therefore inlet passages and outletpassages. The exhaust gases are therefore constrained to pass throughthe lateral walls of the inlet passages into the outlet passages; thusparticles and soot are deposited in the filter body.

After a certain time of use, soot accumulates in the passages of thefilter body, which increases the head loss caused by the filter anddegrades the performance of the engine. For this reason, the filter bodymust be regenerated regularly, conventionally after about 7 to 10 hoursof operation, when the head loss has reached a value of approximately150 dPa (for an engine of about 2 liters cubic capacity driving on amotorway with a filter body of approximately 4 liters).

Regeneration consists in oxidizing the soot. To this end, it isnecessary to heat the soot since the temperature of the exhaust gases isof the order of 300° C. whereas the flash point temperature of the sootis more of the order of 600° C., under normal operating conditions.Despite such regeneration, combustion residues remain in the filterbody. Accordingly, the head loss induced by the filter body afterregeneration is always higher than that induced by the filter bodybefore regeneration. This phenomenon of clogging continues on eachregeneration and it is necessary for the dealer to clean the filterblock completely, for example every 80 000 km. This cleaning constitutesa drawback to the use of filter bodies.

FR 2 473 113 proposes a filter body that can be produced by extrusionand has inlet passages of greater cross section than the outletpassages. The authors indicate a filtering area of the filter unit of7.89 cm²/cm³ (i.e. 0.789 cm²/l) with a constant inlet passage crosssection less than 12.9 mm² and a wall thickness less than or equal to0.7 mm.

However, the filter body described in FR 2 473 113 induces a high headloss, which means that the filter body must be regenerated frequently.It is therefore difficult to envisage the industrial use of this filterbody.

There is therefore a need for a filter body having a low head lossthroughout its service life and therefore necessitating less frequentcleaning. The invention aims to meet that need.

The invention relates more particularly to a filter unit for filteringparticles contained in the exhaust gases of an internal combustionengine, comprising interleaved sets of adjacent inlet passages andoutlet passages, said inlet and outlet passages being in fluidcommunication through their lateral walls, said lateral walls having, incross section, an undulation determined to increase the overall volumeof said inlet passages at the expense of that of the outlet passages,and the overall volume of the inlet passages being greater than that ofthe outlet passages, noteworthy in that:

the hydraulic diameter of said outlet passages is from 0.9 to 1.4 mm,preferably greater than 0.95 mm,

the ratio r of the overall volume of the inlet passages to the overallvolume of the outlet passages is from 1.15 to 4, preferably greater than1.35 and/or less than 3,

the filtering area is from 0.825 m² to 1.4 m² per liter of said filterunit, preferably greater than 0.92 m²

the ratio of asymmetry of said undulation is less than 20%.

As will emerge in more detail hereinafter, this significantly reducesthe head loss induced by the filter unit and therefore reduces thefrequency of regeneration of the filter body of which it forms part.

According to other preferred features of the invention:

said outlet passages have a cross section of constant area throughoutthe length of said filter unit;

said inlet and outlet passages are straight and parallel;

said inlet and outlet passages are arranged relative to each other sothat all of the gas filtered by any inlet passage passes into outletpassages adjacent said inlet passage;

said undulation has a sinusoidal shape in cross section; the ratio ofasymmetry of said undulation is less than 15%, preferably less than 12%,and/or greater than 5%, preferably greater than 6%;

said undulation is periodic and a half-period of said undulation extendsover the width of one of said channels;

said inlet and outlet channels are disposed alternately in anyhorizontal row or vertical row of said unit, thus forming a checkerboardstructure on the front or rear face of the unit.

The invention also relates to a filter body intended for a particlefilter that is noteworthy in that it includes at least one filter unitaccording to the invention.

The following description with reference to the appended drawings andthe examples explain the invention and its advantages. In the drawings:

FIG. 1 a is a partial view of the front face (i.e. that on which theexhaust gases impinge) of a prior art filter unit, FIG. 1 b is a view ofthat unit in section taken along the line AA in FIG. 1 a, and FIG. 1 cis a view in cross section of an extrusion die for producing the abovefilter unit,

FIGS. 2 a to 2 c are views analogous to those of FIGS. 1 a to 1 c,respectively, and show a first embodiment of a filter body of theinvention,

FIG. 3 is a partial view of the front face of a second embodiment of afilter unit of the invention,

FIG. 4 is a graph representing the head loss as a function of the timeof use for various new, “clean”, tested filter bodies, and

FIG. 5 is a graph representing the head loss as a function of the timeof use for various tested filter bodies in which the combustion residuesoccupy a volume corresponding to 50% of the volume of the inlet passagesof the reference filter, which corresponds to a distance traveled by thevehicle of approximately 80 000 km. Such filter bodies are referred toas “clogged”. The residues are generally in the far end of the inletpassage.

All of FIGS. 1 to 3 correspond to partial views of filter units and maybe regarded as a partial view of a monolithic filter body or a partialview of a filter body formed by assembling filter units.

In the figures, the thickness of the walls between the passages is notto scale and is not limiting on the invention.

FIG. 1 a is a diagram of the front face of a filter unit currently usedto trap particles contained in the exhaust gases of motor vehiclespropelled by a diesel engine. This filter unit has identical passageswhose cross section is square and of constant size throughout the lengthof the filter body. On this front face, one in two passages is blocked.The passages 1 and 2 are open and therefore constitute inlet passages.The passages 3 and 4 are blocked and therefore constitute outletpassages. FIG. 1 b is a view in longitudinal section taken along theline AA in FIG. 1 a. The flow F of exhaust gases enters the filter unitvia the inlet passages and then passes through the lateral walls of thepassages into the outlet passages. FIG. 1 c is a view in cross sectionof the extrusion die used to fabricate the filter units used at presentand represented in FIG. 1 a. In this view, the solid lines representmachined openings through which the ceramic can pass.

FIG. 2 a is a diagram of the front face of a first embodiment of afilter unit of the invention. The passages 10 and 11 are open andconstitute inlet passages. The passages 12 and 13 are blocked andconstitute outlet passages. The passages are arranged in an array ofpassages having a triangular cross section that is deformed to increasethe overall volume of the inlet passages at the expense of that of theoutlet passages. Thus a non-plane intermediate wall between an inletpassage and an outlet passage may be concave on the side of the inletpassage, as shown in FIG. 2 a, and convex on the side of the outletpassage.

FIG. 2 b is a view in section taken along the line AA in FIG. 2 a. Theflow F of exhaust gases enters the filter body via the inlet passagesand passes through the walls of the passages into the outlet passages.Because of the increase in the overall volume of the inlet passagesreferred to above, the area available on the walls of the inletpassages, or “filtering area”, is increased to the detriment of that ofthe outlet passages compared to a prior art filter body such as thatshown in FIG. 1.

All of the area of the inlet passages is advantageously used to filterthe exhaust gases, as there are no portions of one or more inletpassages that open into other inlet passages, such portions being of noutility for filtration since the exhaust gases can pass through them inboth directions.

The inlet passages and outlet passes are preferably parallel andstraight. It is therefore possible to produce the filter unit of theinvention by extrusion, which is advantageous.

FIG. 2 c is a view in cross section of the extrusion die used to producethe filter unit shown in FIG. 3 a; in this view the solid linesrepresent machined openings through which the ceramic may pass. This dieis used to fabricate passages of constant cross section throughout thelength of the filter unit, which facilitates extruding them.

The passages are straight along the length of the filter body.Accordingly, in longitudinal section (see FIG. 2 b), the passages have aconstant cross section throughout their length L. This facilitates thefabrication of the filter units.

The inlet passages have a greater cross section than the outlet passagesin order to increase the volume available for storing soot. The inletpassages and the outlet passages are arranged with respect to each otherso that all of the gas filtered by any inlet passage passes into outletpassages adjacent that inlet passage, which optimizes the surface areaavailable for a given filter unit volume.

FIG. 3 is a diagram of the front face of another embodiment of a filterunit of the invention. The passages 10 and 11 are open and constituteinlet passages. The passages 12 and 13 are blocked and constitute outletpassages. The passages are organized in an array of passages having asquare cross section that is deformed to increase the overall volume ofthe inlet passages at the expense of that of the outlet passages. In anyhorizontal row (x) or vertical row (y), the inlet and outlet channelsare disposed alternately, forming a checkerboard structure. The lateralwall 14 of an inlet passage 11 is therefore formed of four lateral wallportions 14 a-14 d separating the interior volume of that passage fromthe interior volumes of the four respective adjacent outlet passages.

A non-plane intermediate wall 15 between two horizontal rows R₁ and R₂,and/or two vertical rows, of passages (and thus formed by a set ofportions of lateral walls 16 ₁ to 16 ₈ of those passages) is preferablyconcave on the side of the inlet passages and convex on the side of theoutlet passages.

Along a horizontal row (along the x axis) or a vertical row (along the yaxis) of passages, the intermediate wall 15 preferably has an undulatingor “wavy” shape in cross section, the wall 15 undulating bysubstantially one half of an undulation length across the width of apassage.

The “length” of an undulation is the distance between two points of theundulation located at the same height with the same direction ofvariation of slope. In the case of a periodic undulation, the “length”of the undulation is called the “period”.

The undulation is preferably periodic, but the amplitude of theundulations may be constant or variable. The amplitude is preferablyconstant. It is also preferable if the undulation has a sinusoidal shapewhose half-period is equal to the pitch “p” of the array of passages, asshown in FIG. 3.

Finally, it is preferable if all the vertical or horizontal intermediatewalls 15 of a unit have an undulation of exactly the same shape in crosssection.

The expression “ratio of asymmetry” refers to the ratio between theamplitude “h” and the half-length of said undulation (or between theamplitude “h” and the half-period in the case of a periodic undulation).The following examples summarized in table 1 are provided by way ofillustration and are not limiting on the invention. FIGS. 4 and 5represent curves of the increasing head loss as a function of timecorresponding to certain examples from table 1, with clean and cloggedfilters, respectively.

The filter bodies that were tested were produced by assembling 16 filterunits fastened together by means of a joint 1 mm thick. These filterbodies were cylindrical with a diameter of 144 mm and a length of 9inches (228.6 mm). The passages were of the type represented in FIG. 4,the walls having a substantially sinusoidal profile and the outlet andinlet passages having a cross section of constant area through thelength L of the filter body.

For the purposes of the calculations, the exhaust gases were introducedinto the inlet passages of the filter bodies under test at a temperatureof 250° C. and a flowrate of 320 m³/hour. The concentration of particlesin the exhaust gases was 2.2*10⁻⁵ kg/m³.

For the clogged filter body tests, the concentration of combustionresidues in the inlet passages was 1.8*10⁻⁹ m³/m³ of exhaust gas.

The reference example “Ref” corresponds to a filter constituted byassembling 16 filter units fastened together with a joint 1 mm thick.This filter was cylindrical with a diameter of 144 mm and a length of 9inches (228.6 mm). The passages were of the type represented in FIG. 1,the outlet and inlet passages having a square cross section of constantarea through the length L of the filter body. The pitch of the array was1.8 mm and the thickness of the walls was 350 μm.

The filtering areas, passage volumes and head losses were calculated bythe Institut de Mécaniques des Fluides of Toulouse (France).

The expression “hydraulic diameter” used in relation to a cross sectionor a passage refers to the ratio between four times the section of thepassage and the perimeter of the passage.

The passage density is expressed as a number of passages per square inch(cells per square inch (cpsi)).

Ve denotes the total volume of the inlet passages, Vs the total volumeof the outlet passages. The ratio r is defined as follows: r=Ve/Vs.

The expression “filtering area” refers to the area of the walls of theinlet passages through which the flow of gas to be filtered can pass.The filtering area is evaluated in square meters per liter of filterunit.

The performance of a filter body is evaluated by measuring the time “t”in minutes to reach a particular head loss “dP” and by the initial headloss (dP for t=0). The measured time “t” in minutes to achieve a headloss “dP” of x mbar is denoted t_(/x).

It is considered advantageous for a filter body to conform to thefollowing criteria:

initial head loss<50 mbar;

t_(/100)≧300 for a clean filter;

t_(/150)≧500 for a clean filter;

t_(/150) ≧200 for a clogged filter. TABLE 1 Outlet Filtering t(min) fort(min) for t(min) for dP passage area per dP = dP = dP = (mbar) PassageWall hydraulic liter of 150 mbar 100 mbar 150 mbar for t = 0 densitythickness Ratio of r = diameter filter unit (clean (clean filter(clogged (clean (cpsi) (μm) asymmetry Ve/Vs (mm) (m²/l) filter unit)unit) filter unit) filter unit) Ref. 200 350 0 1 1.45 0.918 481 319 13426.7 Ex1 250 350 0 1 1.26 0.997 495 341 NA 28.7 Ex2 250 350 10% 1.9861.03 1.149 >600   443 279 33.9 Ex3 250 350 20% 4.806 0.74 1.283 522 100NA 89.6 Ex4 250 300 10% 1.867 1.09 1.183 >600   508 337 27.8 Ex5 250 30015% 2.66 0.97 1.25 >600   514 NA 36.8 Ex6 250 300 20% 4.061 0.821.314 >600   376 NA 60.5 Ex7 250 400 10% 2.099 0.97 1.115 >600   372 NA41.8 Ex8 200 350 10% 1.883 1.2 1.05 >600   412 264 29.7 Ex9 200 350 15%2.723 1.06 1.111 >600   411 309 38.7 Ex10 200 350 20% 4.223 0.891.168 >600   281 284 63.8 Ex11 300 350 10% 2.054 0.9 1.233 >600   446 NA39.5 Ex12 200 350  2% 1.132 1.4 0.946 516 342 160 26.3 Ex13 200 350  5%1.365 1.33 0.987 566 374 200 26.6 Ex14 100 500 10% 1.367 1.88 0.696 307180 121 35.7 Ex15 150 400 10% 1.363 1.54 0.856 441 288 200 28.8“NA” means “not available”.

Table 1 and FIGS. 4 and 5 indicate that:

For new filter bodies, the greater the filtering area, the slower thehead loss increases over time. In other words, the loading slopedecreases as the filtering area increases. However, the filtering areais not the only criterion, as is shown by comparing example 15 and thereference example. That comparison shows that, according to theinvention, a higher ratio r has the advantage of compensating a lowerfiltering area when the filters are clogged.

Without being bound by any theory, the Applicant explains thisphenomenon in the following manner.

A high ratio r means a greater volume in the inlet passages for storingthe combustion residues. For a given filtering area and a givencombustion residue volume (i.e. a given number of regenerations), theproportion of the filtering area that is ineffective because it iscovered by the combustion residues is therefore lower. The induced headloss is therefore lower. Between two regenerations, the head lossinduced by the filter body therefore increases more slowly.

Moreover, the large volume in the inlet passages can store a greaterquantity of combustion residues. The number of regenerations beforeremoving/refitting the filter can therefore be increased.

For a constant wall thickness, an increase in the ratio of asymmetryimplies an increase in the storage capacity of the inlet passages and anincrease in the filtering area of the unit.

However, the ratio of asymmetry must not be increased excessively, asthis could reduce the section of the outlet passages to the point of aprejudicial increase in the head loss.

A compromise must therefore be arrived at. The ratio of asymmetry isless than 20%, preferably less than 15%, more preferably less than 12%,and greater than 5%, preferably greater than 6%.

According to the invention, the time between two filter bodydemounting/cleaning operations is therefore increased not only becauseof reduced residual clogging of the filtering area by combustionresidues after each regeneration, which slows down clogging by sootbetween two regenerations, but also because a greater number ofregenerations is possible, the combustion residue storage volume beinggreater.

The motorist can therefore travel a greater distance without performingany maintenance on the filter.

According to the invention, the optimum is considered to be having:

a ratio r greater than or equal to 1.15, preferably greater than 1.35,and less than 4, preferably less than 3,

a filtering area at least equal to 0.825 m² per liter of filter unit,and preferably greater than or equal to 0.92 m² per liter of filterunit.

The inlet and outlet passages having a cross section of constant areathroughout the length of the filter unit, the increase in the ratio r isthe result of increasing the hydraulic diameter of the inlet passagesand/or reducing the hydraulic diameter of the outlet passages. Table 1(see in particular examples 3, 6 and 10) shows that if the hydraulicdiameter of the outlet passages is very small, the head loss induced bythe clean filter body is much too high. This may prove unacceptablesince the official power rating of an engine takes account of theexhaust line.

According to the invention, the hydraulic diameter of the outletpassages must be greater than or equal to 0.9 mm and preferably from0.95 to 1.4 mm.

Of course, the present invention is not limited to the embodiments shownhere and described above, which have been provided by way ofillustrative and nonlimiting example.

Thus the invention relates equally to a monolithic filter body. Thefilter unit could have any shape and any arrangement of the passages.

Finally, the cross section of the passages is not limited to the shapesdescribed.

1. Filter unit for filtering particles contained in the exhaust gases ofan internal combustion engine, comprising interleaved sets of adjacentinlet passages (10, 11) and outlet passages (12, 13) in fluidcommunication through their lateral walls, said unit including a set oflateral wall portions (16 ₁-16 ₈) forming an intermediate wall (15)between inlet passages (10, 11) and outlet passages (12, 13) and having,in cross section, an undulation determined to increase the overallvolume of said inlet passages (10, 11) at the expense of that of theoutlet passages (12, 13), and the overall volume (Ve) of said inletpassages (10, 11) being greater than that (Vs) of said outlet passages(12, 13), characterized in that: the hydraulic diameter of said outletpassages (12, 13) is from 0.9 to 1.4 mm, the ratio r of the overallvolume (Ve) of the inlet passages (10, 11) to the overall volume (Vs) ofthe outlet passages (12, 13) is from 1.15 to 4, the filtering area isfrom 0.825 m² to 1.4 m² per liter of said filter unit, the ratio ofasymmetry of said undulation is less than 20%.
 2. Filter unit accordingto claim 1, characterized in that the hydraulic diameter of said outletpassages (12, 13) is greater than 0.95 mm.
 3. Filter unit according toeither claim 1 or claim 2, characterized in that said ratio r is greaterthan 1.35.
 4. Filter unit according to any one of the preceding claims,characterized in that said ratio r is less than
 3. 5. Filter unitaccording to any one of the preceding claims, characterized in that thefiltering area is greater than 0.92 m² per liter of said filter unit. 6.Filter unit according to any one of the preceding claims, characterizedin that said outlet passages (12, 13) have a cross section of constantarea throughout the length (L) of said filter unit.
 7. Filter unitaccording to any one of the preceding claims, characterized in that saidinlet passages (10, 11) and outlet passages (12, 13) are straight andparallel.
 8. Filter unit according to any one of the preceding claims,characterized in that said inlet passages (10, 11) and outlet passages(12, 13) are arranged relative to each other so that all of the gasfiltered by any inlet passage (10, 11) passes into outlet passages (12,13) adjacent said inlet passage (10, 11).
 9. Filter unit according toany one of the preceding claims, characterized in that the ratio ofasymmetry of said undulation is less than 15%.
 10. Filter unit accordingto any one of the preceding claims, characterized in that the ratio ofasymmetry of said undulation is less than 12%.
 11. Filter unit accordingto any one of the preceding claims, characterized in that the ratio ofasymmetry of said undulation is greater than 5%.
 12. Filter unitaccording to any one of the preceding claims, characterized in that saidundulation is periodic and a half-period of said undulation extends overthe width of one of said channels (10, 11, 12, 13).
 13. Filter unitaccording to any one of the preceding claims, characterized in that saidundulation has a sinusoidal shape in cross section.
 14. Filter bodyintended for a particle filter, characterized in that it includes atleast one unit according to any one of the preceding claims.